The invention disclosed and claimed herein is generally directed to a method and apparatus for selectively saturating fat in a band or region in a subject of magnetic resonance (MR) imaging. More particularly, the invention pertains to selectively positioning a fat saturation band with respect to a water saturation band for applications such as two-dimensional time of flight angiography.
Two-dimensional time of flight MR angiography is among the most widely used of all MR angiographic techniques. Often it is desired to image or visualize blood flow through either arteries or veins, but not both at the same time. To achieve this, a "traveling" spatial saturation (SAT) pulse is applied at one side or the other of each of a stack of slices oriented along an axis. Often the slices are oriented substantially axially, in which case their spatial saturation band is applied either superior (cephelad) or inferior (caudal) to the imaging slice. For example, if it is desired to image the flow of blood through an artery, respective slices are selected such that blood flows through each slice. Prior to exciting a given slice for imaging, a SAT band is applied to a region spaced slightly apart from the slice, located on the outflow side of the slice. The saturation pulse prevents blood flowing toward the opposite side of the given slice, i.e., blood flowing through veins, from being detected in the MR imaging process, so that such flow does not interfere with the angiography process directed to the artery. Conversely if the aim is to image blood flow in the vein, the SAT pulse may be placed so as to suppress arterial flow.
While a SAT pulse is generally intended to saturate water molecules in a region close to a slice of interest, a band of fat tissue may also be saturated thereby. If the SAT (water) band is placed very close to the slice, e.g., within 5-10 millimeters, fat suppression can result, which is desirable since it increases blood vessel to background contrast. However, fat suppression in this application has tended to be unreliable, since fairly precise placement of the SAT pulse is required to ensure that the band in which fat is saturated either overlaps or nearly overlaps the slice of interest. Moreover, the exact location of the fat saturation band may be difficult to predict, since it is affected by the sign or polarity of the selection gradient field used in the SAT pulse sequence. In a commercial MR scanning machine, the polarity of the selection gradient, is determined by geometry factors such as the direction of the main magnetic field, the direction of patient entry into the machine (i.e. head first or feet first), and the direction of the gradient coil windings. Because of these factors, it can be very difficult to ensure that the band or region in which fat saturation occurs will be sufficiently close to the slice of interest to be of any benefit in suppressing fat from an acquired image.